Two of the most critical challenges in developing an effecfive AIDS vaccine are to understand how to induce durable immunity at mucosal sites and to define the eariy interactions between HIV and the immune system. In Project 3 we will address both of these issues using the rhesus macaque (RM) model of SIV infection. The precise mechanism by which an AIDS vaccine can confer protective immunity is not known but there is now a general consensus that an effective vaccine should induce both the cellular and humoral arms of the immune system, in particular at the mucosal sites. Recent data from the RV-144 AIDS vaccine trial in Thailand suggest a potential role for antibody in protection from HIV infection. One striking finding from the RV-144 trial was that protection was short-lived and was mostly seen during the first year after vaccination. Thus, a major goal in developing an effective vaccine is to understand how to generate long-lived mucosal immunity. (5D4 T follicular helper (TFH) cells are critical for generating potent and long-lasting anfibody responses and in this project we will determine their role in generating protective humoral immunity following vaccination and infecfion. In our specific aim 1, we will test the hypothesis that long-term humoral immunity is critically dependent on CD4 TFH cells and that the efficient generation of these cells is an essenfial and obligatory component of an effective HIV vaccine. We will examine the magnitude, quality and persistence of SIV specific CD4 TFH cells in both peripheral and mucosal sites after vaccination and determine its correlation to B cell responses. These studies will examine CD4 TFH responses in RMs immunized with DNA/MVA vaccines adjuvanted with GM-CSF (Project 1) and SIV protein immunizafions adjuvanted with TLRs encapsulated in nanoparticles (Project 2). These studies will define which vaccine regimen is most effective in inducing CD4 TFH cells in peripheral and mucosal sites and how the different adjuvants modulate the TFH response and their influence on functional quality and longevity of humoral immunity. In our specific aim 2, we will test the hypothesis that HIV/SIV immunopathogenesis normally precludes the development of a broadly neutralizing anfibody response, but this can be counteracted by immunizafion to generate strong TFH responses before infection. We will determine the suscepfibility of TFH celts to SIV infection and study the infiuence of vaccinafion on their status following infecfion, and the relafionship between the level of virus replicafion in CD4 TFH cells and the main features of SIV infection, including the immune response to the virus and the progression to AIDS.